Part A deals with the evapotranspiration from the reference surface, the so-called reference crop evapotranspiration or reference evapotranspiration, denoted as ETo. The reference surface is a hypothetical grass reference crop with an assumed crop height of 0.12 m, a fixed surface resistance of 70 s m-1 and an albedo of 0.23. The reference surface closely resembles an extensive surface of green, well-watered grass of uniform height, actively growing and completely shading the ground. The fixed surface resistance of 70 s m-1 implies a moderately dry soil surface resulting from about a weekly irrigation frequency.

ETo can be computed from meteorological data. As a result of an Expert Consultation held in May 1990, the FAO Penman-Monteith method is now recommended as the sole standard method for the definition and computation of the reference evapotranspiration. The FAO Penman-Monteith method requires radiation, air temperature, air humidity and wind speed data. Calculation procedures to derive climatic parameters from meteorological data and to estimate missing meteorological variables required for calculating ETo are presented in this Part (Chapter 3). The calculation procedures in this Publication allow for estimation of ETo with the FAO Penman-Monteith method under all circumstances, even in the case of missing climatic data.

ETo can also be estimated from pan evaporation. Pans have proved their practical value and have been used successfully to estimate ETo by observing the water loss from the pan and using empirical coefficients to relate pan evaporation to ETo. However, special precautions and management must be applied.